Media Recognition – Floppy Disks part 1

When archivists receive a collection one of the first things they’ll need to do is identify any digital material. However it won’t be enough to single out for instance all the floppy disks or the CDs – there are subtler distinctions that need to be made. Archivists might not be aware of all these distinctions (or might need reminding) so I’ve been writing a media recognition guide to help archivists determine the specific types of data storage device they may come across. I thought I’d share some of this work with you and first up we have: Floppy Disks.

Sizes

Floppy disks come in four sizes. 8” and 5.25” disks are encased in bendable plastic, 3.5” disks (which are actually 90 x 94mm) are encased in rigid plastic and have sliding metal shutters and 3” disks also come in rigid plastic which is harder than the 3.5” covering and are thicker than the other disk.

Low Level Format

Floppy disks are either single sided (SS) or double sided (DS) and this is often stated on the manufacturer’s label, if the disk has one. Being SS or DS affects a disk’s capacity, but the bigger factor is the low level format, or density of a disk. There are 5 possibilities:

Single Density (SD) – First type of floppy disk available.

Double Density (DD) – On average twice as many bits per time unit can be encoded compared to data on a single density disk, thereby doubling the capacity.

Quad Density (QD) – Holds four times as much data as a single density disk.

High Density (HD) – Further expands disk capacity allowing 5.25” disks to store up to 1,200 KB and 3.5” disks to store up to 1,440 KB of data

Extended Density (ED) – Enlarges disk capacity to 2,880 KB

Not all sizes of disk are compatible with the different densities, which is important to know when identifying the disk type.

Size and number of sides used

Possible Densities

Capacity

8 inch single sided

Single

250 KB

8 inch double sided

Single

500 KB

Double

1.2 MB

3 inch single sided

Single

180 KB

Double

360 KB

3 inch double sided

Single

360 KB

Double

720 KB

5.25 inch single sided

Single

80 KB

Single

90 KB

5.25 inch double sided

Single

180 KB

Double

320 KB

Double

360 KB

Quad

720 KB

High

1,200 KB

3.5 inch single sided

Double

360 KB

High

720 KB

3.5 inch double sided

Double

720 KB

High

1,440 KB

Extended

2880 KB

Writing to Disk Methods

There are two ways in which data is written to a floppy disk. The tracks on floppy disks are smaller on the inside of the disk than on the outside, therefore when writing data the disk can either maintain a constant speed in terms of the time taken for the disk to rotate 360 degrees or in terms of the time taken to cross each track. Constant Angular Velocity (CAV) maintains a constant rotation speed whereas Constant Linear Velocity (CLV) ensures that each track is crossed at the same speed.

CAV is used by most microcomputer platforms, such as IBM PC, Amstrad CPC and Microsoft Windows, but Apple adopted CLV. This method is more efficient, thus explaining why Apple disks have larger capacities. However, CLV requires a special mechanism, which renders Apple floppy disks incompatible with other computers. Apple eventually abandoned this method and adopted the CAV standard.

8 Inch Disks

Type:

Magnetic storage media

Introduced:

1971

Active:

No

Cessation:

Quickly declined in popularity after the 5.25” disk was introduced and by 1978 most manufacturers had adopted 5.25” disk drives.

Capacity:

500 KB – 1.2 MB

Compatibility:

Contemporary machines had inbuilt floppy drives. Drives can be externally attached to modern computers.

Users:

Almost universal during the height of their popularity in the 1970s: was one of only a few affordable mass storage devices available. Used by individuals and small organisations for data storage and backup and by manufacturers for booting software.

File Systems:

CP/M most common. Also ODS-1

Common Manufacturers:

Disks: IMB, Maxell, Shugart Associates, 3M

Drives: IMB, Shugart Associates, Burroughs Corporation

Recognition

The 8 inch floppy was the first disk to be introduced in 1971. Like the other disk sizes, 8 inch floppy disks consist of a disk with a central hole encased in a plastic envelope.

The easiest way to identify the low level format of an 8” floppy disk is by any labelling on the disk. The majority of manufacturers brand the disk as double density if that is what it is. The format ‘Single Sided’ was not coined until Double Sided was invented in 1976, therefore early SS disks will not be labelled as such. However, this does not mean all unspecified disks are single sided. Likewise it is difficult to determine the density of a disk if this is not labelled, although the capacity of the disk is a good indicator.

High Level Formatting

All files written to a floppy disk are contained within a file system. The specific file system used depends on the disk manufacturer, but 8” disks will generally contain either ODS-1 or CP/M

8 inch Disk Drives

The first read-write 8” floppy drives were introduced in 1972 with a capacity of 175 KB. In 1973 IBM released its own floppy disk drive which used a different recording format thereby increasing the capacity to 250 KB. This became known as Single Sided Single Density (SSSD) and drives supporting SSSD were soon being made by several manufacturers. 8” floppy disk drives were the storage device of choice used by the first ‘microcomputer’ operating system in the 1970s. Double sided drives were first introduced by Burroughs Corporation in 1975 with a 1 MB capacity.